Creation of Ferromagnetic Properties in Ni-Doped Na2WO4—Effect of Hydrogenation Post-treatment

Abstract

Pure and Ni-doped sodium tungstate (Na2WO4) powders were synthesised by simultaneous crystallisation method. The effects of Ni doping on the structural, optical, and magnetic properties of the host Na2WO4 powder were studied. The study of X-ray diffraction shows that the incorporated Ni ions occupy locations in interstitial positions and substitution for W ion in the Na2WO3 lattice. A monophase cubic structure was obtained when the as-crystallised Na2WO4 powder was doped with Ni ions or annealed in hydrogen gas atmosphere (hydrogenation). The optical properties were studied by diffuse reflectance spectroscopy (DRS) technique. It was established that the direct bandgap of Na2WO4 exhibits red shift from 4.6 to 3.50 eV with Ni doping and blue shift to 5.13 eV with hydrogenation. The purpose of the present study is to study conditions necessary to prepare powders having room-temperature ferromagnetic (RT-FM) properties. Therefore, the Na2WO4 nano-powder was doped with Ni ions. RT-FM properties were obtained with Ni-doped Na2WO4 that was strongly enhanced by hydrogenation so that the energy product (EP) was increased by 213 %. This enhancement was attributed to the enhancement of the magnetic medium for the spin-spin (S-S) interaction inside the crystalline medium. In general, an experimental relationship was established between O vacancies, optical absorption, and magnetic properties of the studied crystal. Thus, it was proved, for the first time, the possibility of producing Na2WO4 having RT-FM, where magnetic characteristics can be tailored by doping and post-treatment under H2 atmosphere, thus a new potential candidate to be used in magnetic applications of ferroelectric crystals.